attachment_icon │ filter-hook │ WP 1.0

Usage

add_filter( 'attachment_icon', 'wp_kama_attachment_icon_filter', 10, 2 );

/**
 * Function for `attachment_icon` filter-hook.
 * 
 * @param  $icon 
 * @param  $ID   
 *
 * @return 
 */
function wp_kama_attachment_icon_filter( $icon, $ID ){

	// filter...
	return $icon;
}

$icon

-

$ID

-

Where the hook is called

return apply_filters( 'attachment_icon', $icon, $post->ID );

Where the hook is used in WordPress